The present invention relates to the field of mass spectrometry and more particularly to an ion mirror for a time-of-flight (TOF) mass spectrometer. The invention provides an ion mirror that is integral to a flight tube of a TOF mass spectrometer.
Ion mirrors are often used in mass spectrometers to reflect the out going ion stream back towards the detector and by so doing, reduce the physical length of the flight chamber, while maintaining the desired flight path and providing energy compensation. In a typical TOF mass spectrometer, ions are generated in an ion source, accelerated into a field free region, and then eventually sent to a detector. In order to obtain high instrument resolution, a narrow range of arrival times of isobaric ions is important. Ions with the same mass to charge ratio (m/z) should have the same arrival times. Most importantly, ions can start from different source locations and this can affect overall resolution. It is, therefore, necessary in some cases to correct the velocity variations of these differing ions. In particular, U.S. Pat. No. 5,994,695 discloses an apparatus for manipulating ions that includes a flexible substrate and a conductive material for manipulating ions. The invention includes a xe2x80x9cstackxe2x80x9d of plates for producing electric fields that retard the ions as they pass through the apparatus. This design or xe2x80x9cstackxe2x80x9d has been used in the mass spectrometry field for producing improved resolution in mass spectrometers. In addition, steps have been taken to reduce the number of plates in xe2x80x9cstacksxe2x80x9d to provide for more efficient apparatus design with improved ion resolution. In particular, mirrors with only three-cylindrical elements have been designed that achieve improved off-axis homogeneity compared with other conventional simple geometry mirrors (Zhang and Enke, Jour. Of Am Soc. Mass Spect., 2000, 11, 759-764).
As discussed above, when the time spent in the mirror is optimally adjusted, all ions with the same m/z arrive at the detector at the same time despite differences in kinetic energy. A number of attempts have been made to develop a mirror that will provide the best means of producing consistent arrival times. Theoretically, the ideal type of instrument would be a xe2x80x9cperfectronxe2x80x9d that could correct arrival times over a large or diverse range of kinetic energies (A. L. Rockwood 34th AMS 1986). Perfectrons have a quadratic axial potential distribution (Vx=ax2), where Vx is the axial potential at any depth x, and a, is a constant. The total flight time is defined as Ttotal=kmxc2xd and is only proportional to the square root of m/z. Perfectrons, however, suffer from a number of limitations including lack of field free ion drift regions, and the need for many electrodes throughout the drift region""s separation of ion source, ion mirror and ion detector. In addition, ideal field shape has not been obtained with an ion beam with finite width.
Other kinds of xe2x80x9ctime-focusingxe2x80x9d arrangements subject the ions to time-varying fields that have the effect of decelerating the faster ions and accelerating the slower ions with the aim of equalizing the flight times of all ions having the same mass. None of these known time-focusing arrangements is completely effective and, in practice, the flight times of ions that have the same mass still exhibit an energy dependency, and thus reducing the mass-resolving power of the spectrometer.
In addition to the mirrors discussed above, a number of effective non-ideal mirrors have been designed. These mirrors can be classified as being both linear and non-linear according to the electric field distribution along the mirror axis. One type of ion mirror subjects the ions to a static electric field, and an example of this is the xe2x80x9creflectronxe2x80x9d, described by B. A. Mamyrin, V. I. Karatev, D. V. Schmikk and V. A. Zagulin in Soviet Physics JETP 45, 37 (1973). The reflectron subjects the ions to a uniform electric field in two regions so as to cause their deceleration and reflection. The more energetic ions penetrate deeper into the field region than the less energetic ions. With a suitable choice of field parameters, it is possible to arrange that ions having different energies, but the same mass, arrive at a detector at closely the same time. A gridded element orthogonal to the mirror axis is used to separate each linear electric field from the others. It should be noted, however, that the larger the difference of field gradients on either side of the grid, the more likely the opportunity for mirror distortions. Although these linear ion mirrors are generally easier to fabricate, they still lack the mass resolution that can be obtained with non-linear mirrors.
The above-described mirror designs suffer from a number of problems. For instance, the plates used in the apparatus may often be spaced imprecisely, or are not aligned appropriately. It would be desirable, therefore, to provide an apparatus in which each of the electrodes can be easily aligned and mounted with permanent high precision and accuracy. It would also be desirable to provide a mirror in which the electrodes that are used in the ion beam transmission are few in number and are integral to the ion mirror substrate or flight tube and do not need to be fabricated separately. These and other problems present in the prior art have been obviated by the present invention.
In general, the invention provides an ion mirror for a mass spectrometer. The ion mirror is integral to a flight tube of a mass spectrometer and comprises a front electrode, middle electrode and a rear electrode. The three electrodes comprise a conductive material and are designed for receiving ions and creating an electric field that retards and reflects ions. The flight tube may comprise an insulating material such as fused silica or a quartz material.